How to fk up perfectly good jetting (two stroke)?

Related:
Create New Tag

9/24/2018 2:32 PM

I'm trying to get a better handle on how jetting works in theory by better understanding what it takes to fuck up otherwise perfect jetting.

Say for instance I have a brand new 250 two stroke with spot on jetting. No spooge out of the silencer, nice and crisp running bike.

How could I make spooge come out of the exhaust? (with only touching carb jetting and nothing else)

Would I want to increase or decrease the main jet size? Or, would I want to raise or lower the clip?

|

Ride like a girl!!!
2016 KTM 500 EXC-F
Current project: 2007 Kawasaki KX250-R7 retro build
Current project #2: 1987 Kawasaki KX250-E1 rider/racer resto
1989 KX125 resto
1987 RM250 resto

9/24/2018 2:46 PM

You would add bigger pilot, & main and drop the needle position if you’d want to make it richer. Not sure why you would want to do that

|

2011 Yamaha YZ 250 -MX
1996 Yamaha Banshee 350 -Trails
1982 Chevy S10
2005 Pontiac GTO -451rwhp

9/24/2018 5:43 PM

Over-oil your filter or decrease the amount of oil in your gas. Or putt-putt around not getting the combustion temps up.

|

Powerband in every gear !

9/24/2018 9:02 PM

Raising the needle (lower the clip) will richen it up and could increase spooge.
Turning air screw inward will richen idle speed. Dirty air filter.

These bikes were meant to be raced and not blow up. A little spooge is ok like on the exhaust tip.
Dripping black spooge along your swingarm is not ok.

|

9/25/2018 1:51 AM
Edited Date/Time: 9/25/2018 1:52 AM

Raise float level and jetting gets richer

|

9/25/2018 6:31 AM

I like your reverse engineering thought process..

|

Positively, absolutely 110% obsessed with anything MOTO.

9/25/2018 12:00 PM

Let's aid your knowledge by telling you what happens in your carburetor:

Airflow enters your carb throat at some velocity, based on engine RPM and throttle opening.
The airflow "picks up" fuel vapor from the jets via the venturi effect. The jets meter out a precise amount based on the size of the holes bored in them. The result is there is a fuel/air mixture which travels onward through the reed valves and into the engine.


Starting at closed throttle, there is very little airflow. so little, in fact, that the pilot jet is the only real source of fuel. Barely cracking the throttle allows a little more air through, which picks up more fuel from the pilot. This is called your "slow circuit" or pilot circuit. At such small throttle openings, the slide is most or all of the way down and the jetting needle is resting on the main jet, blocking its opening.

At middle throttle settings, there is more airflow and more venturi effect. Fuel vapor is starting to get sucked out past the tip of the needle through the main jet. Note that the flow from the main jet is still blocked partially by the needle, but some fuel is getting sucked out past the tip of the needle. This is why the needle tip and taper is so important.

As the throttle gets opened near wide open or fully open, the needle jet is finally raised above the main jet so it can flow its maximum amount of fuel. Note that the "fully open" main jet comes into play when the needle leaves its seat on the main jet, which can be adjusted by the needle position clip.

Thus:
The pilot jet only affects the fuel/air mixture at low throttle apertures, (about 0 to 1/4 throttle,)
The needle affects middle apertures (about 1/4 to 3/4 throttle,)
And together, the needle position and main jet affect throttle settings from about 1/2 to full throttle.

Lager numbers on the jets equate to more fuel and a richer condition. Vice-versa for smaller numbers.
Moving the clip down makes the needle go up, allowing more fuel out of the main jet earlier, thus making a richer condition. (Vice-versa for moving the clip up.)






|

Braaapin' aint easy.

9/25/2018 12:46 PM

Falcon,

Your analysis is very good! I would add that the pilot circuit is always functioning from idle to WOT. It is most effective of course from 0-1/8th or so throttle but its always flowing fuel. I like to think of the total carb fuel flow as:

0-1/8th = pilot circuit fuel flow only
1/16th-1/4 = slide cutaway and straight section fuel flow + pilot fuel flow
1/4-3/4 = needle clip/needle taper fuel flow + pilot fuel flow (starting to reach max fuel flow of pilot jet)
3/4 - WOT = main jet only (but.... if needle end diameter is too large, the needle is limiting the fuel at WOT not the main as it should) + little fuel flow from pilot as the velocity is too great and pilot starts to choke off)

I have always notice how the air screw settings do affect well into the throttle openings and I was curious to see what was going on. I have been able to clean up almost a half clip on the needle at 1/2 throttle with an air screw adjustment which the carb circuit diagrams say isn't possible. I have also seen some jet kits come with needles that the tip end is so large, it restricts full flow of the main jet. People get very confused with the jetting. lol

To the OP, this is how I learned to jet years ago. I had a bike where the jetting was perfect and I sat for hours mixing and matching jets and listening to what sounded lean and rich and basically screwed it up then went back to good jetting. It helped me a ton!! I like your thinking!

|

9/25/2018 10:15 PM

more spooge request? less air (make a duct tape shield over some of the air filter)

If it starts spooging and you switch it to PERFECT, the oil still in the pipe and silencer will still blow out for a while. This is something some of us don't think about.

I think almost all of us do not have a crystal clean pipe and silencer to go along with any jetting change.

|

9/26/2018 5:12 AM

Falcon wrote:

Let's aid your knowledge by telling you what happens in your carburetor:

Airflow enters your carb throat at some velocity, based on engine RPM and throttle opening.
The airflow "picks up" fuel vapor from the jets via the venturi effect. The jets meter out a precise amount based on the size of the holes bored in them. The result is there is a fuel/air mixture which travels onward through the reed valves and into the engine.


Starting at closed throttle, there is very little airflow. so little, in fact, that the pilot jet is the only real source of fuel. Barely cracking the throttle allows a little more air through, which picks up more fuel from the pilot. This is called your "slow circuit" or pilot circuit. At such small throttle openings, the slide is most or all of the way down and the jetting needle is resting on the main jet, blocking its opening.

At middle throttle settings, there is more airflow and more venturi effect. Fuel vapor is starting to get sucked out past the tip of the needle through the main jet. Note that the flow from the main jet is still blocked partially by the needle, but some fuel is getting sucked out past the tip of the needle. This is why the needle tip and taper is so important.

As the throttle gets opened near wide open or fully open, the needle jet is finally raised above the main jet so it can flow its maximum amount of fuel. Note that the "fully open" main jet comes into play when the needle leaves its seat on the main jet, which can be adjusted by the needle position clip.

Thus:
The pilot jet only affects the fuel/air mixture at low throttle apertures, (about 0 to 1/4 throttle,)
The needle affects middle apertures (about 1/4 to 3/4 throttle,)
And together, the needle position and main jet affect throttle settings from about 1/2 to full throttle.

Lager numbers on the jets equate to more fuel and a richer condition. Vice-versa for smaller numbers.
Moving the clip down makes the needle go up, allowing more fuel out of the main jet earlier, thus making a richer condition. (Vice-versa for moving the clip up.)






That is probably the most straight forward and best explained description of jetting I have ever read. Thanks

|

9/26/2018 2:54 PM
Edited Date/Time: 9/26/2018 2:55 PM

Falcon wrote:

Let's aid your knowledge by telling you what happens in your carburetor:

Airflow enters your carb throat at some velocity, based on engine RPM and throttle opening.
The airflow "picks up" fuel vapor from the jets via the venturi effect. The jets meter out a precise amount based on the size of the holes bored in them. The result is there is a fuel/air mixture which travels onward through the reed valves and into the engine.


Starting at closed throttle, there is very little airflow. so little, in fact, that the pilot jet is the only real source of fuel. Barely cracking the throttle allows a little more air through, which picks up more fuel from the pilot. This is called your "slow circuit" or pilot circuit. At such small throttle openings, the slide is most or all of the way down and the jetting needle is resting on the main jet, blocking its opening.

At middle throttle settings, there is more airflow and more venturi effect. Fuel vapor is starting to get sucked out past the tip of the needle through the main jet. Note that the flow from the main jet is still blocked partially by the needle, but some fuel is getting sucked out past the tip of the needle. This is why the needle tip and taper is so important.

As the throttle gets opened near wide open or fully open, the needle jet is finally raised above the main jet so it can flow its maximum amount of fuel. Note that the "fully open" main jet comes into play when the needle leaves its seat on the main jet, which can be adjusted by the needle position clip.

Thus:
The pilot jet only affects the fuel/air mixture at low throttle apertures, (about 0 to 1/4 throttle,)
The needle affects middle apertures (about 1/4 to 3/4 throttle,)
And together, the needle position and main jet affect throttle settings from about 1/2 to full throttle.

Lager numbers on the jets equate to more fuel and a richer condition. Vice-versa for smaller numbers.
Moving the clip down makes the needle go up, allowing more fuel out of the main jet earlier, thus making a richer condition. (Vice-versa for moving the clip up.)






chump6784 wrote:

That is probably the most straight forward and best explained description of jetting I have ever read. Thanks

Thanks. Glad to help!

The the OP: I should have added the following to answer your question:
Switch to a larger main jet and/or lower the clip (raises the needle) to make your bike start spooging.

|

Braaapin' aint easy.

9/26/2018 6:05 PM

8 ounces of oil to a gallon of gas should get things brewin'.

|

9/26/2018 7:10 PM
Edited Date/Time: 9/26/2018 7:11 PM

By the time you get enough fuel in it to cause crap to spew or simply drool out the end of your silencer, your bike will be so rich that it will run like crap.

|